As a geology graduate, echoing in part what others have said:
-Go on as many field trips as possible; take advantage of any student conferences (i.e. SIMEW and SIFT in Canada) that you can
-Enroll in as many field courses as possible; even do geophysics field school if you are in geology.
-Pick up as many electives as possible; it will broaden your geological perspective in ways you may not be able to forsee.
-Read as many papers as possible; you will understand only a fraction at first, but the learning curve will quickly propel you forward.
-Take as many writing courses as you can, and write often. Even start a blog to hone your skills.
-Present as often as possible. It can be extremely painful and stressful, but it will be very beneficial.
-Start a dialog with your professors very early; don't pester them for answers like every other undergrad, but ask them legitimate questions
-Work in private industry for the summer. Not only will you make make excellent money, but it will be akin to taking an extra course.
-Don't limit yourself to either hard rock or soft rock, but don't hold yourself back either if you have a clear preference.
-If you have the opportunity to do an honour's thesis, DO IT.
-If you are looking to get into private industry, make sure you have all the science courses for professional registration (i.e. P.Geo status in Canada).
-Take the time to learn the 'boring' or 'hard' stuff (i.e. optical properties, structural geology, isotopes) - it'll pay off big-time eventually.